Similar to batteries, fuel cells function to produce electricity through chemical reactions. Rather than storing reactants as batteries do, fuel cells are operated by continuously supplying reactants to the cell. In a typical fuel cell, hydrogen gas acts as one reactant and oxygen as the other, with the two reacting at electrodes to form water molecules and releasing energy in the form of direct current electricity. The apparatus and process may produce electricity continuously as long as hydrogen and oxygen are provided. While oxygen may either be stored or provided from the air, hydrogen gas may be generated from other compounds through controlled chemical reactions rather than storing hydrogen, which may need to be compressed or cryogenically cooled. As fuel cell technology evolves, so do the means by which hydrogen gas is generated for application with fuel cells.